This invention relates to methods and apparatus for transmitting optical signals in an optical transmission system with optimum power while avoiding degradation of the optical signal due to nonlinear processes occurring in an optical transmission medium constituting a component of the optical transmission system.
It is generally desirable in optical transmission systems for optical signals to be transmitted via optical waveguides of the system at high power levels in order to maintain sufficient signal to noise ratios over extended transmission distances such that an acceptably low level of bit error rate is present in the received optical signal. Optical waveguides such as optical fibres however comprise an optical transmission medium which exhibits nonlinear effects at high power levels, resulting in degradation of the optical signal. Nonlinear effects may similarly occur within optical terminals of the system in optical transmission media of components such as optical amplifiers. The optimum power level at which optical signals could be transmitted is typically the maximum power level at which significant degradation due to nonlinearity is avoided. Since the performance of various optical components within the system having respective optical transmission media will vary with operating conditions and age or component replacement, a safety margin is allowed in setting the maximum power level when designating the system. Consequently, it is typically the case that optical transmission systems will operate at power levels which are less than the optimum power level.
A further problem is that an individual optical component forming part of the optical transmission system may suffer a sudden or gradual loss of performance but without total failure such that unacceptable degradation of system performance characterised by the presence of the products of a nonlinear process occurs. Often it may then be difficult to locate the faulty component.
Of particular concern in considering nonlinear processes are the effects of four wave mixing, particularly in relation to WDM (wavelength division multiplexed) optical transmission systems having a number of frequency channels separated by a substantially uniform frequency channel spacing such that each frequency channel may potentially become corrupted with the product of four wave mixing associated with respective pairs of other frequency channels.
In the example shown in FIG. 2(a), four channels having frequencies xcfx891, xcfx892, xcfx893 and xcfx894 are transmitted with equal power.
Considering photon interactions between the xcfx892 and xcfx893 channels only, the four wave mixing process may take the form
xcfx891=2xcfx892xe2x88x92xcfx893=xcfx892xcex94xcfx89xe2x80x83xe2x80x83Equation 1;
or
xcfx894=2xcfx893xe2x88x92xcfx892=xcfx893+xcex94xcfx89xe2x80x83xe2x80x83Equation 2;
where xcex94xcfx89 is the frequency separation between channels.
The products of four wave mixing between photons in channels xcfx892 and xcfx893 therefore occur at frequencies xcfx891 and xcfx894 as illustrated in FIG. 2(b), resulting in a loss of power from the channels xcfx892 and xcfx893 and interference in the wavelength bands around xcfx891 and xcfx894.
For an analysis of nonlinear processes the reader is referred to xe2x80x9cNonlinear Fiber Opticsxe2x80x9d, Second Edition, Govind P. Agrawal, 1995 and in particular to Chapter 10 which deals with four-wave mixing. Problems associated with four-wave mixing are discussed in U.S. Pat. No. 5,410,624 which proposes using dissimilar wavelength channel spacings to avoid interference. The onset of nonlinearity with increasing levels of optical power is discussed in U.S. Pat. No. 5,420,868 which proposes amplitude and phase modulation of components of an optical beam to suppress Stimulated Brillouin Scattering. The cumulative effect of all degradations in a system can be determined by eye measurements as described in U.S. Pat. No. 4,823,360. The individual sources of degradation cannot however by distinguished.
It is known from U.S. Pat. No. 5,512,029 to modulate the optical signals such that each single wavelength channel is modulated with a high speed data stream and a respective low speed, small amplitude dither signal. The dither signals are mutually orthogonal pseudo-random sequences which can be reliably identified by digital correlation techniques in a performance monitoring apparatus. By monitoring a received optical signal, the performance of components of an optical transmission system in terms of the effects of random noise processes is measured by comparing the modulation depth in the decoded dither signal with the known modulation depth of the transmitted dither signal. Defects in components of the system may thereby be detected by observing changes in signal to noise measurements obtained by monitoring the dither modulation depth.
This technique however does not provide any specific sensitivity to the products of nonlinear processes. Furthermore, none of the above references provide a satisfactory technique for monitoring and controlling nonlinear processes within an optical transmission system.
It is an object of the present invention to provide apparatus and method for monitoring optical signals in an optical transmission system to detect degradation of the optical signal due to nonlinear processes.
It is a further object of the present invention to provide a method of controlling the optical power of an optical signal so as to avoid the onset of nonlinear processes.
According to the present invention there is disclosed an optical transmission system comprising;
a plurality of optical terminals;
optical waveguide means interconnecting the optical terminals and operable to conduct optical signals therebetween; and
at least one of the waveguide means and the optical terminals comprising an optical transmission medium susceptible to a nonlinear process at high power levels of the optical signal;
wherein at least one optical terminal comprises monitoring means operable to monitor degradation of the optical signals consistent with the nonlinear process occurring in the optical transmission medium and to output monitored data representative of such degradation.
In the context of the present invention, the term optical terminal is to be understood to include transmitters, receivers, repeaters or other processing elements to which the waveguide means are connected, and in particular including elements of the system which comprise optical amplifiers.
Preferably the optical terminal comprises an alarm indicating means responsive to the monitored data and operable to provide an alarm indication indicative of such degradation being detected.
It is thereby possible to provide the system with one or more alarms which will alert an operator to the need to undertake investigative or remedial action, possibly be replacing a component containing the optical transmission medium upstream of the monitoring means.
The alarm indicating means may provide a remote or local alarm.
The monitored data may also be input to a control means operable to control the power level of the optical signal. When nonlinear processes are detected, the power level may be reduced until the monitored data no longer indicates the presence of products of the nonlinear process. The integrity of the transmitted data may therefore be maintained, subject to the effects of operating at a reduced power level.
Preferably the monitoring means is operable to detect products of four wave mixing. Where the optical signals are wavelength division multiplexed with a plurality of single frequency channels separated by a substantially uniform frequency channel spacing, the monitoring means may be operable to detect products of four wave mixing in at least one of the single frequency channels.
Since prevailing data transmission standards typically require substantially uniform frequency channel spacing, a particular problem is that the products of four wave mixing cause interference and cross-talk in adjacent frequency channels. The monitoring of such processes to provide an alarm or power control feedback therefore provides significant advantages in preserving the integrity of the transmitted data and channel separation.
According to a preferred embodiment, an optical transmission system comprises signal modulating means operable to modulate the single frequency channels with respective high speed data streams and dither modulation means operable to modulate the single frequency channels with respective low speed, small amplitude dither signals.
An advantage of such dither modulation is to provide each of the transmitted frequency channels with a marker which can be identified by a correlation process in the received optical signals in the event of there being cross-talk or cross-channel interference resulting from four wave mixing.
Preferably the monitoring means comprises sampling means operable to detect an optical signal sample of the optical signal after having been transmitted through the optical transmission medium and further comprising a processor operable to perform a correlation between the optical signal sample and reference data representative of dither induced modulation in the four wave mixing product to obtain a correlation value constituting said monitored data.
Preferably the reference data comprises at least one spectral template derived from a predetermined pseudo-random sequence encoded in a respective dither signal.
According to a further aspect of the present invention there is disclosed a method of controlling the optical power of an optical signal in an optical transmission system; the method comprising the steps of;
conducting an optical signal in an optical transmission medium susceptible to a nonlinear process at high power levels of the optical signal;
determining the optical power of the optical signal by action of a variable gain device;
monitoring the optical signal by operation of a monitoring means so as to detect degradation of the optical signal consistent with a nonlinear process occurring in the optical transmission medium;
outputting monitored data from the monitoring means representative of such degradation; and
controlling the variable gain device by operation of a control means responsive to the monitored data such that the level of degradation as indicated by the monitored data remains within a predetermined limit.
According to a further aspect of the present invention there is disclosed monitoring apparatus for use in an optical transmission system comprising sampling means operable to sample a received optical signal to provide sampled data, a processor operable to correlate the sample data with reference data representative of a product of a four wave mixing process being present in the received optical signal; and means for outputting monitored data representative of the detection of products of the four wave mixing process.